Recently, in order to reduce the weight of various components for the improvement of fuel efficiency of a vehicle, an application of a reduction in thickness by the strengthening of a steel sheet such as an iron alloy; and a light metal such as an Al alloy is progressed. However, compared to a heavy metal such as steel, a light metal such as an Al alloy has an advantage of high specific strength and a disadvantage of having a significantly higher cost. Therefore, the application is limited to specific uses. Therefore, in order to reduce the weight of various components at a lower cost over a wider range, a reduction in thickness with the strengthening of a steel sheet is necessary.
Generally, the strengthening of a steel sheet brings about a deterioration in material properties such as formability (workability). Therefore, in the development of a high-strength steel sheet, it is important to increase strength without impairing material properties. In particular, for a steel sheet which is used for vehicle components such as inner plate components, structural components, and suspension components, bendability, stretch flangeability, burring workability, ductility, fatigue resistance, impact resistance (toughness), corrosion resistance, and the like are required according to its use. Therefore, it is important to achieve a high level of balance between these material properties and high strength.
In particular, among automobile components, components which are processed using a sheet material as a base metal and function as a rotator, such as, a drum or a carrier constituting an automatic transmission are important components for transmitting engine output to axle shafts. In order to reduce friction and the like, circularity as a shape and homogeneity in thickness in a circumferential direction are required for these components. Furthermore, since a forming processes such as burring, drawing, ironing, and stretching are used for these components, ultimate deformability which is represented by local elongation is significantly important.
In a steel sheet used for these components, it is preferable that impact resistance (toughness), which is the property of a component to be difficult to fracture when being attached to a vehicle after formation and then being impacted by collision or the like, is improved. In particular, when use in a cold region is taken into consideration, in order to secure impact resistance at a low temperature, it is preferable that the toughness at a low temperature (low-temperature toughness) is improved. This toughness is defined by vTrs (Charpy fracture appearance transition temperature). Therefore, it is important to increase the above-described impact resistance of a steel material.
That is, in a thin steel sheet for components which require homogeneity in thickness and include the above-described components, in addition to superior workability, it is required that both plastic isotropy and toughness are simultaneously improved.
Techniques for improving both high strength and various material properties such as formability are as follows. For example, Patent Document 1 discloses a method of producing a steel sheet in which a steel structure contains 90% or greater of ferrite and the balance consisting of bainite; and thus high strength, ductility, and hole extensibility are simultaneously improved. However, regarding a steel sheet which is produced according to the technique disclosed in Patent Document 1, Patent Document 1 does not disclose plastic isotropy at all. Therefore, for example, assuming that this steel sheet is applied to a component, such as a gear wheel, which requires circularity and homogeneity of thickness in a circumferential direction, there is a concern about power reduction by inappropriate vibration or friction loss due to a misaligned component.
In addition, Patent Documents 2 and 3 disclose a high-tensile hot-rolled steel sheet having high strength and superior stretch flangeability in which Mo is added for refining precipitates. However, in a steel sheet which is produced according to the techniques disclosed in Patent Documents 2 and 3, since it is necessary that 0.07% or greater of Mo, which is an expensive alloy element, is added, there is a problem of high production cost. Furthermore, the techniques disclosed in Patent Documents 2 and 3 do not disclose plastic isotropy. Therefore, assuming that this steel sheet is applied to a component which requires circularity and homogeneity in thickness in a circumferential direction, there is a concern about power reduction by inappropriate vibration or friction loss due to a misaligned component.
Meanwhile, regarding the improvement of plastic isotropy of a steel sheet, that is, the reduction of plastic anisotropy, for example, Patent Document 4 discloses a technique in which endless rolling and lubrication rolling are combined to control an austenite texture of a surface shear layer and thus to reduce the in-plane anisotropy of r values (Lankford values). However, in order to perform such lubrication rolling having a low friction coefficient over the entire coil, endless rolling is necessary for preventing engagement failure caused by a slip between a roll caliber tool and a rolled material during rolling. Therefore, in order to apply this technique, there is a large burden because facilities such as a rough bar joining apparatus or a high-speed crop shear are required.
In addition, for example, Patent Document 5 discloses a technique in which a combination of Zr, Ti, and Mo is added; and finish rolling is finished at a high temperature of 950° C. or higher to reduce the anisotropy of r values at a strength of 780 MPa grade or higher and thus to improve both stretch flangeability and deep drawability. However, since it is necessary that 0.1% or greater of Mo which is an expensive alloy element, is added, there is a problem of high production cost.
Furthermore, although techniques of improving the toughness of a steel sheet have been progressed in the related art, a hot-rolled steel sheet having high strength and superior plastic isotropy, hole expansibility, and toughness is not disclosed in Patent Documents 1 to 5.